1. Field of the Invention
The present invention relates to an implantable heart-stimulating device with which it is possible to stimulate both the ventricles of a heart, i.e. a bi-ventricular pacemaker or defibrillator (pacer).
The invention also relates to a system including such a device and to the use of the system.
2. Description of the Prior Art
Several different implantable devices for stimulating a heart are known. Such devices normally are able to sense electrical activity of the heart. Some implantable devices are able to deliver stimulation pulses to both the left and right ventricles of the heart, and sometimes also to the left and right atria.
Devices that are able to deliver stimulation pulses to both the left and right ventricles are called bi-ventricular pacers. Such devices can be used to treat patients who suffer from different severe cardiac problems, e.g. patients suffering from congestive heart failure (CHF). CHF is defined generally as the inability of the heart to deliver a sufficient amount of blood to the body. CHF can have different causes. For example, it can be caused by a left bundle branch block (LBBB) or a right bundle branch block (RBBB). By using bi-ventricular pacing, the contraction of the ventricles can be controlled in order to improve the ability of the heart to pump blood. The stimulation pulses to the two ventricles can be delivered simultaneously, but it is also known to deliver the stimulation pulses to the two ventricles with a short time delay between them in order to optimize the pumping performance of the heart.
U.S. Pat. No. 5,720,768 describes different possible electrode positions in order to stimulate or sense the different chambers of the heart.
U.S. Pat. No. 6,070,100 describes positioning the electrodes in both the left and the right atrium as well as in the left and the right ventricles.
In connection with implantable pacers, in particular pacers which only have the possibility to stimulate the right ventricle, and sometimes also the right atrium, it is known to detect capture of the heart, i.e. to detect whether the heart actually reacts to a delivered stimulation pulse. If the heart is not captured the pacer delivers a back-up pulse with a higher pulse energy than the first pulse. It is also possible to increase the pulse energy of future stimulation pulses if capture is not detected. In order to save battery capacity it is important that the stimulation pulses are not delivered with an unnecessarily high energy. By varying the energy of the stimulation pulses and by detecting capture it is possible to find a threshold value for the stimulation pulse energy. Based on the threshold value, a suitable stimulation pulse energy can be determined.
The detection of capture involves several problems. Different signals from the heart or generated by the pacemaker may interfere with each other, which may make the detection of capture difficult. The evoked response that it is intended to detect may thus be hidden because of other electrical phenomena. It is particularly difficult to detect capture in a bi-ventricular pacer, because in such a pacer there are more delivered and detected signals which may interfere with each other.
U.S. Pat. No. 6,148,234 describes a system for detecting capture in connection with bi-ventricular or bi-atrial pacing. This patent describes the fact that if a chamber is captured, there is a biological refractory period during which this chamber cannot be stimulated again. The system described in this patent monitors these refractory periods for the different chambers, for example for the two ventricles. When capture is achieved in both ventricles, no intrinsic depolarization signals can be sensed during the following refractory period. However, where the output level of one of the pacing pulses is insufficient to capture one ventricle, but capture is achieved in the other ventricle, a delayed depolarization pattern can be detected in the ventricle that was not captured. This delayed depolarization is due to an interventricular conduction from the ventricle that is captured to the ventricle that is not captured. The system according to this patent thus monitors the refractory interval following each delivery of stimulating pulses to the ventricles. A loss of capture is indicated in case such a delayed depolarization is sensed during the refractory period.
Also United States Patent Application Publication 2001/0049542 describes a system for detecting capture in connection with bi-ventricular or bi-atrial stimulation. The system includes a morphology detector incorporated in a micro-controller to allow for the processing of the sensed intra-cardiac electrogram signals (IEGM). The morphology of the IEGM may depend on whether both the ventricles (or atria) have captured or not. By detecting the shape of the IEGM capture may thus, be detected.
Another concept used in this technical field is “fusion”. Fusion may occur when an intrinsic depolarization of the heart takes place simultaneously, or substantially simultaneously, with a stimulation pulse from the heart-stimulating device.